Prevention of overloading in speech circuits



Aug. 25, '1925.

1,550,684 L. EsPENscHar-:D

REVENTION OF OVERLOADNG N SPEECH CIRCUITS Clt Patented fitug. Z5, 19215.

LLYD ESPENSCHIED, F HOLLIS, NEVI YORK, ASSIGNOR T0 AMERICAN TELEPHONE TELEGRAPH COMPANY, A CORPEATTN OF NEW YORK.

PREVENTTON 0F OVER/LOADING 1N SPEECH CTRGTTS.

Appicaton tiled. September 19, 1924. Serial No. 738,716.

To all whom t may concern.'

Be it known that T, LLOYD EsrnNsoi-inn, residing` at Hollis, in the county ot Queens and State of New York, have invented certain Tmprovements in the Prevention of Gverloading in Speech Circuits, of which the following is a specilication.

This invention relates to transmission circuits, and more particularly to a method of and apparatus for preventing distortion effects due to overloading ot vacuum tubes in such circuits.

Tn connection with the use of vacuum tubes in repeater and radio circuits, it is desirable to keep the volume of speech transmission below a value which would overload the tubes and cause distortion. Where the volume is subject to great variations, as is the case in connection with the pick-up ot radio broadcast material, it is very important to control the volume that is applied to a vacuum tube. At has hitherto been the practice to take care of this matter through the manual adjustment et the volume applied to the tube by an operator who watches a so-called level indicator.

The ellect of overloading the vacuum tubes in a speech circuit is to produce distortion of the transmission. rThis distortion may be regarded as taking two general forms; (a) a chopping otl et the peaks in the speech wave due to the current limiting characteristic ot the tube (this action may also be regarded as one in which the various frequency components comprising speech are ampliiied to different degrees), and (b) the production of intermodulation frequencies due to the interaction et the component trequencies present simultaneously in the speech, these intermodulation frequencies arising owing to the tact that the amplifying circuit when overloaded is operating as a modulating device.

Tt has been proposed to employ the saturation characteristic of the vacuum tube as a means for limiting the power passing over a circuit. This may readily be accomplished to a certain extent in telegraphy due to the use of a single transmission frequency, but it cannot be practiced in telephony because, it the volume of the speech currents is sutciently great to exceed the saturation point of the tube, the same kind of distortion will occur in the output circuit of the power limiting tube as results from the overloading of the amplier tubes in their r-egular operation.

Tt is the purpose of the present invention to provide a method, and instrumentalities for practicing the method, which will permit ot the transmission of telephonie currents through power limiting tube without intermodulation oit the frequencies, so that the vohime transmitted to the amplifying or other tubes ot the system will not overload such other tubes. Tn fact, it desired, the method may be used directly in connection with the amplifyi i tubes themselves to prevent the intermodulation due to the overloading` of such tubes.

The invention may new be more fully understood from the following description thereof when read in connection with the accompanying drawing, the figure of which indicates schematically a circuit arrangement embodying the principles of the invention.

As already stated, distortion due to overloading may be considered as resulting from intcrmodulation in the overloaded tube between the various component frequencies and the voice transmission. Thus an 800- cycle component may modulate with a 1200- cycle component. Tn so doing not only are these two components reduced in amplitude, but new components are originated. Prominent among these components are the sum and difference terms (2000 cycles and 400 cycles) and their harmonics. The depreciation in quality is due in large part to these spurious components. There is also some distortion due to the cutting oftq of the peaks in the speech wave shape, but experiments have shown that considerable distortion oi this type may be allowed to take place in a telephone circuit without affecting the articulation seriously. Consequently, it the depreciation in quality due to the intermodulation eiect can be eliminated, it will be possible to obtain high quality transmission.

In order to accomplish this result, in accordance with the present invention the telephone band is divided by means of filters into a plurality of sub-bands, each sub-band being less than one octave in width. Thus,

' nent-s, as will loe described later. rThe seve it the voice band started with 100 cycles, the sub-bands could be chosen as Yfollows:

Sub-band No.

Frequency limits For exa mp1 e The ligure oit the drawing' shows the manner in which. these bands would be separately impressed upon a power lin'iiting; tube and then recombined to 'torni the voice baud. ML designates the transmission line carrying the voice baud7 and this line is divided into a number oit branches L, la, etc., each provided with an input baud liltcr such as F1, F2, etc., lier the purpose ot' dividing' the band transi'nitted by the line Pelli into ay plurality of sub-bands, a diilii'erent sub-band being' transmitted to each branch. A vacuum tube amplitier will be included in each branca as indicated at nl, A2, etc. VlFliese vacuum tube ampli tiers may talze the place ot the regular amplilier oit the system or may be power limiting tubes ttor protecting the regular ampliliers occurring' at later' points iu the system. ln the output circuits et the ampliliers of the various branches additional filters such as FQ, FQ, etc.. are provided Ator filtering` out the intermodulation compobranches L1, L2, etc., are merged in the outgoing' transmission line ML.

To understand how such au arrane'ement overcomes intermodulation, it should be noted trom the table above given that the highest frequency in any sub-band is less than double the lowest frequency so that the lowest sub-compoi'ient result-ing;` trom inter-v modulation ot the components ot the subband will be greater 'than the highest Ytrequency, and the highest dilierence 'Frequency resulting' from intermodulat-ion will be lower than the lowest Yfrequency ot the sub-band. For example, consider two components lying' within the first subband, such as 120 and 150 cycles. These frequencies would pass through the tilter F, and be excluded by all oit' the other channel tilters. It overloading occurs in the tube JL, the sum and difference 'frequencies would be 270 cycles and 30 cycles, respectively, and as they both lie without this hand, neither will pass through the output 'filter FQ. lilre'v'ise, the harmonics oit Q are all greater than 180. which is the upper limit oit the band, and will not pass through the filter FQ. rllhe harmonics of 30 which fall below l00 and above 180 will also be excluded, although those falling; within the band will not be eliminated. It is thus seen that practically all extraneous frequencies which may be produced can be kept out of the output circuit.

By the use ot au arrangement such as that above described1 the output resulting iirom recombining the sub-bands will always be ol5 goed quality independent et the volume iuput. For low input volumes the output rolume will, of course, vary with the input, and the distorting action will not occur, but when any component of the input exceeds the volume level which the tubes in the various branches can handle, the volume output will cease to rise but will rcn'iain substantially constant at the maximum level. 'lf'he arrangement there tore constitutes an automatic volume limiting; device, and the organization illustra-ted in the ligure may be inserted as a whole in any transmission sys-- tem7 including' vaccum tubes used as ampli-- tiers and the like, to linut the volume which will be transmitted to the tubes', or the arrangement maj,r be siibstitutedA 'tor any aniplilier of the system, in which case the tilters acting' in combination with. the annaliliers eliminate the undesired iii /qiscncies resulting' 'trom intermodulation due to overloading the am'. iliers.

in any practical case it will. be possible to reduce the number ot sulidivisions required by talzi advantage oit the 'tact that the lower 'frequency portion ot the voice rango contains most oit the voice energy, and thus overloading would always occur as in the lower sub-bancs. Another advantage ot the arrangement is that it may be also used iter equalization ot the transmission at diliercnt trequencies. The gain oit each et the vaccum tubes Al, A2. etc., may be adjusted in a welllfnouf'n manner so that the grain ot each amplilier n'iaf.' be so set as to compensate ilior the circuit losses at the 'frequencies involved.

lt will be obvious that 'the general principles herein disclosed may be embodied in many organizations widely di tlierent trom those illustrated without departing' trom the spirit ot the invention as delined in the 'tollowing' claims.

1What is claimed is:

l. The method ot preventing' distortion duc to the intermodulation oit component sipjnal frequencies arising' 'from the transmission over a transmission circuit ot signal currents having amplitudes ,great enough to overload a translating' device in the circuit` which consists in obtaining 'lrom the band ot signal 'frequencies a band the upper limiting' frequency ot which is lower than the second harmonic oit 'the lower limiting; trequency, limiting` the energy transmitted through a. portion ot said circuit to an a mount less' than the peak value oit' the band, thereby producing' intermodulation components, selecting; from the resultant treoueneies a band oit 'frequencies coinciding' with the band transmitted, and suppressing lil i,55o,csa

the intermodulation components lying outside said band.

2. The method of preventing distortion due to the intermodulation or component signal Yfrequencies arising from the transmission over a transmission circuit or signal currents having amplitudes great enough to overload a translating device in the circuit, Which consists in separating the band into a plurality ot sub-bands', each having its upper limiting frequency less than the second harmonic or its lower limiting lirequency, transmitting each sub-band through a separate path, limiting the energy transmitted through certain ot the paths to an amount less than the peak value ot the band, thereby producing intermodulation components, selecting from each. path only the frequencies corresponding to the original sub-band, and combining the selected sub* bands to form a band corresponding to the original bane.

3. ln a signaling system, a transmission circuit including translating devices adapted, when overloaded, to produce intermodulation components, means to transmit over said circuitsignal currents having peak amplitudes great enough to cause over* loading ot said translating devices, an arrangement to prevent the distort-ion due to such overloading comprising means to produce a band or frequencies 'from the original signal frequencies having an upper limiting frequency which is lovver than the second harmonic or the lower limiting treqiiiency, means to limit the energy ot said band transmitted through a portion of said circuit to an amount less than the peak value ot the band, thereby producing interinodulation components, and means to select from the resultant frequencies a band coinciding with the band originally produced, While suppressing intermodulation components lying outside said band.

4L. ln a signaling system, a transmission circuit including translating devices adapted, when overloaded, to produce intermodulation components, means to transmit over said circuit signal currents having peak amplitudes great enough to cause overloading ot said translating devices, an arrangementto prevent the distortion due to such overloading comprising means to select from the band cii signal Jvtrequencies to be transmitted a plurality of sub-bands, each having its upper limiting frequency less than the second harmonic oi its lov-Jer limiting frequency, means to limit the energy transmitted by certain of said sub-bands, thereby producing intermodulation components, means "r'or selecting from each energy-limiting arrangement a band corresponding to the original sub-band, and means for combining the selected sub-bands to form a band corresponding to the original band.

ln a signaling system adapted for the transmission oi' voice currents, the energy of which fluctuates through a considerable range, and including vacuum tubes adapted to be overloaded when the signal energy eziceeds a limiting value, an arrangement for preventing the distortion due to such overloading comprising a plurality or branch circuits, selective devices in each branch circuit for selecting from the normal signal band a sub-band, the upper limiting requency ot Which is less than the second harn monic of its lower limiting frequency, a power li? device in each branch to limit the en gl, in the branch, thereby producing intermodulation When the energy of the band exceeds the limit, and a second selective device in each branch tor selecting 'from the resultant trequencies a band corresponding to the sub-band originally selected, thereby suppressing intermodulation componentslying outside said sub-band.

ln testimony whereof, l have signed my naine to this specification this 18th day of September 1924.

Lnorn nsrnnscrrinn. 

